This invention relates generally to a chain hoist. More particularly, but not by way of limitation, this invention relates to an improved load limiting apparatus for a chain hoist.
It is highly desirable to be able to prevent an overload being imposed on a chain hoist. It is desirable, not only because of possible damage to the hoist, but also because of possible injury to the person using the hoist. Accordingly, it has been found convenient to provide a chainwheel or drivewheel in the hoist that includes some means for permitting rotational or relative rotational movement between the portion of the chainwheel being driven by the chain, and the hub portion that is connected to the drive shaft of the hoist. Such relative rotation occurs when an over load exists and prevents the hoist from lifting the load.
The foregoing is illustrated in the following U.S. Pat. Nos.: 3,724,815 issued Apr. 3, 1973 to Harold W. Hawkins, et al; 3,770,086 issued Nov. 6, 1973 to Harry Y. Jackson et al; and 3,953,000 issued Apr. 27, 1976 to William R. Zorbaugh. In the '086 patent, relative motion is permitted when a particular torque load is imposed due to a plurality of spring loaded balls that are carried by the hub and ride in recesses provided in the outer chain driven rim of the hoist. The balls are spring-loaded in slots and are urged against the rim, preventing such relative motion, until the load imposed is adequate to depress the springs and displace the balls from the recesses.
The '815 patent illustrates a very similar mechanism, but in this patent, rollers are mounted on lever arms that are pivoted on the hub. The rollers are disposed in spaced recesses formed in the outer rim of the chaindrive mechanism while the inner ends of the lever arms are spring loaded to bias the rollers into the recesses provided. Accordingly, the mechanism rotates as one unit until the torque load exceeds an amount sufficient to overcome the springs and move the rollers out of the recesses provided.
In the '000 patent, relative motion between the outer rim and hub is prevented by a plurality of friction discs located relatively between the two parts. The friction discs are biased by a resilient member against a "brake" plate so that relative rotation cannot occur until the frictional force exerted thereby is exceeded by the torque load imposed on the hoist.
From the foregoing, it can be seen that the same basic approach is used in each of the patents, but the precise mechanism used therein is entirely different. Each of the foregoing will function and each is relatively complex, and relatively expensive to manufacture.
Accordingly, an object of this invention is to provide an improved load limiting apparatus for a hoist that is relatively simple in structure, easy and inexpensive to manufacture, and easily maintained.